A human vertebra has a rearwardly projecting portion known as a spinous process. Bending or the natural aging and degeneration of the spine can cause the adjacent vertebrae and their spinous processes to be moved toward each other. This constricts the space in the spinal canal and foramina and, thus, may cause pain. Such constriction, known as stenosis, can be treated by the use of an implant in the space between adjacent spinous processes.
Generally speaking there are two types of spinal stenosis: (1) hard or rigid spinal stenosis, or (2) soft or dynamic spinal stenosis. In both cases, spinal stenosis may be caused by excessive growth of tissue due to degeneration, loss of disc height, as well as disorders such as spondylolisthesis where the normal relative position and/or orientation of the adjacent vertebrae have been modified.
The most significant difference between the two types of spinal stenosis is generally that dynamic spinal stenosis may be treated with distraction of the vertebra at the affected level while hard stenosis generally requires removal of the tissue that obstructs the spinal canal or foramina at the affected level. In case of tissue removal, the patient generally must accept some loss of stability of the spine. Therefore, it is preferable to increase the stability of the spinal segment by inserting an interspinous spacer between adjacent vertebrae to increase the stiffness of the segment and/or to restrict motion of that segment. Additional stability may be desirable and may be accomplished by adding plates to rigidly fix the spacer to the spinous processes and eliminate motion at that segment (i.e. fusion).
Many spinous process plate systems have a rigid construction that limits their ability to adapt to the varying anatomy of the spinous processes. The large size of many plate systems also limits their use to levels with sufficiently large spinous processes, making them unusable at L5-S1. Additionally, some plate systems consist of multiple pieces which require assembly in-situ. As a result, they sometimes require the simultaneous use of several instruments making the surgical technique and visibility into the operation site difficult. Still other plate systems do not offer an interspinous spacer component to help limit extension of the spine and further stabilize the segment.
Current instrumentation that works with existing spinous process plates requires significant lateral muscle retraction. For instance, in order to lock the plates of the implant to the spinous processes, many instruments compress the implant plates from the outside. This requires additional lateral dissection and muscle retraction to accommodate additional space for the instrument which leads to longer recovery time and healing for the patient. Furthermore, because the forces required to compress the plates can be high, these locking (crimping) instruments can become extremely big and bulky making it difficult for the surgeon to see into the wound. To improve visibility, some systems use smaller instruments but require the simultaneous use of two crimpers instead of one. Some systems require multiple instruments and steps to insert and lock the implant in place.
Thus, there is a need for a spinous process plate system that comes pre-assembled with an interspinous spacer, adapts to the bony anatomy for a wider range of spine levels, and can be easily fixated to the spine. In addition, there is a need for an instrument that can easily insert, crimp, and allow locking of a spinous process plate with minimal lateral muscle retraction while still allowing visualization of the implant.